In LTE (Long Term Evolution), RLC (Radio Link Control) layers on a transmitting side and receiving side are configured to execute window controls to provide an order control and a duplicate control.
In the window control of the RLC layer on the transmitting side, an SN (Sequence Number) is assigned to an RLC-PDU (Radio Link Control-Protocol Data Unit) to be newly transmitted. Also, the RLC layer on the transmitting side updates “Tx window (transmitting side window)” or performs retransmission in the RLC layer depending on an RLC-status-report (ACK/NACK) from the RLC layer on the receiving side.
On the other hand, for the window control of the RLC layer on the receiving side, “Rx window (receiving side window)” is updated when successfully receiving the RLC-PDU in order of the SN.
If failing to receive the RLC-status-report from the RLC layer on the receiving side, the RLC layer on the transmitting side cannot update “Tx window (transmitting side window)” nor transmit a new RLC-PDU due to the occurrence of “Tx window stalling”.
To avoid this, the RLC layer on the receiving side needs to transmit the RLC-status-report to the RLC layer on the transmitting side at proper frequency.
Note that the RLC layer on the receiving side is configured to transmit the RLC-status-report in a case such as receiving a feedback request (which is transmitted periodically or at the end of RLC-PDU) from the RLC layer on the transmitting side, or detecting an SN being lost from the received RLC-PDUs.
In this connection, the RLC layer on the receiving side is configured not to transmit the RLC-status-report when “prohibit timer” is activated.
Such “Prohibit timer” is configured to be activated at a time of the transmission of the RLC-status-report.
In Release-12 of LTE, “Inter-eNB CA (Carrier Aggregation)” in which CA is executed between different radio base stations eNB has been under discussion and consideration in order to improve a throughput (for example, see non-patent document 1).
Additionally, an architecture shown in FIG. 6 has been under discussion and consideration as one of U-plane architectures in such “Inter-eNB CA”.
In such architecture, a PDCP (Packet Data Convergence Protocol) layer function of a radio base station MeNB (Master-eNB) distributes downlink data addressed to a mobile station UE to a radio base station SeNB (Secondary-eNB), thereby achieving improvement of a throughput of such mobile station UE. Note that such distribution is referred as “Bearer split”.
In such architecture, it should be noted that the PDCP layer function on the mobile station UE side needs to perform order correction processing on two RLC (Radio Link Control) streams.